Silicone wafer cleaning method

ABSTRACT

The present invention provides a method for cleaning a silicon wafer with a cleaning fluid, comprising 35 to 65% by weight of HNO 3 , 0.05 to 0.5% by weight of HF, 0.05 to 0.5% by weight of HCl, 0.002 to 0.1% by weight of a surface-active agent, and water. The silicon wafer cleaning method involves comprising treating the surface of a silicon wafer with said cleaning fluid. According to the present method, etching of the silicon wafer surface can be carried out simply with the amount of the etching being controlled to several tens Å, and particularly about 20 to 30 Å, and without damage to the smoothness of the surface. In addition, contamination with gold and other heavy metals of the order of 10 12  atoms/cm 2  can be decreased to not more than 1/100.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a silicon wafer cleaning fluid that isuseful, for example, in the process of producing semiconductor siliconwafers and in the process of producing semiconductor devices and to asilicon wafer cleaning method using said cleaning fluid.

2. Description of the Prior Art

As cleaning fluids that are widely used in removing intensecontamination with heavy metal contaminants on the surface ofsemiconductor silicon wafers, for example, a fluid which consists ofhydrochloric acid, hydrogen peroxide, and water and is called SC-2(hereinafter referred to as SC-2) used in the RCA method, and oxidizingcleaning fluids, such as nitric acid and aqua regia, are known. Thecleaning of silicon wafers using these cleaning fluids is generallycarried out such that silicon wafers are supported by fluororesincarriers and are dipped in these cleaning fluids for a prescribedperiod. When silicon wafers are dipped in the above cleaning fluids,heavy metals on the silicon wafer surface are dissolved and removed.However, with respect to the effect of removing heavy metals by thesecleaning fluids, the examination of the contamination with gold that isthe least removed among heavy metals reveals that it is recognized thataqua regia and SC-2 are effective in cleaning intense contamination withgold amounting approximately to 10¹⁵ atoms/cm² or more, but the cleaningeffect is not satisfactory if the contamination is not as high as thislevel and a cleaning effect can not be obtained if the contamination ison the order of 10¹³ atoms/cm² or below. This is because if thecontamination is 10¹⁴ atoms/cm² or less, at the time of cleaning, aspontaneous oxide film is formed on the wafer surface and gold atoms aretaken under the oxide film.

Therefore, it is desirable that while the spontaneous oxide film beremoved such that an oxidizing agent can be effective. The presentinventor proposed in Japanese Patent No. 613521 a cleaning fluidcomprising 1 volume of aqua regia and 0.3 volume or less of HF as anagent for removing the spontaneous oxide film. According to the methoddescribed in the above patent, it is shown that contamination with goldamounting to 10¹⁵ atoms/cm² can be reduced by one cleaning treatment to10¹⁰ atoms/cm². However, the etching action on the silicon wafer surfacecaused by the HNO₃ and HF is strong because it is facilitated bychlorine and nitrosyl chloride generated from the aqua regia and at acertain concentration of HF, this cleaning fluid etches the siliconwafer in an amount of 1 μm or more in some cases, and the fineindentations (haze) generated on the silicon wafer damages the specularstate of the surface. If the cleaning is effected with the HFconcentration of the cleaning fluid greatly lowered to an extent wherehaze will not be generated, because a small difference in the generatedamount of nitrosyl chloride and chlorine causes the etching amount tovary considerably, the conditions of the cleaning become hard tocontrol. Thus, this cleaning fluid is not suitable for the process ofproducing IC, LSI circuits, etc.

Therefore, where particularly intense contamination with heavy metals isrequired to be removed, a treatment of dissolving heavy metals with SC-2or aqua regia and an etching treatment of removing the spontaneous oxidefilm with dilute hydrofluoric acid are alternately repeated. However,although this cleaning method can remove metals that can be easilyoxidized, such as copper, the method cannot exhibit satisfactorycleaning effect on contamination with gold and, for example, even if thedilute hydrofluoric acid/SC-2 treatment is repeated 10 times,contamination with gold is reduced only to about 1/2.

On the other hand, in the process of producing semiconductor devices,there is a step of giving strong, though shallow defects to the siliconwafer surface, such as reactive ion etching (RIE) and ion implantation.Since generally heavy metals are high in diffusion speed, they penetrateeasily into such a defect region and therefore, in that step, there is afear that intense contamination with heavy metals will occur.Accordingly, for such contamination, there is required a cleaning fluidthat can etch and remove the defect region itself. On the other hand,the etching should not damage the smoothness of the silicon wafersurface. Thus, a cleaning fluid and cleaning method that can control theetching to a thickness of several tens Å, and particularly about 20 to30 Å, are required. This is because if the etching is to this extent,the smoothness is not damaged. Further to attain high productivity,desirably it is demanded that the cleaning can be carried out by simplecontact treatment for a short period of about 1 min.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a siliconwafer cleaning fluid and a silicon wafer cleaning method that allow thesilicon wafer surface to be etched simply with the etching amountcontrolled to about several tens Å and can remove or reduce gold andother heavy metals to 10¹⁰ atoms/cm² or less.

The present invention provides, as means of attaining the object, asilicon wafer cleaning fluid, comprising 35 to 65% by weight of HNO₃,0.05 to 0.5% by weight of HF, 0.05 to 0.5% by weight of HCl, 0.002 to0.1% by weight of a surface-active agent, and water. The presentinvention also provides a silicon wafer cleaning method, comprising thestep of bringing the surface of silicon wafers into contact with theabove cleaning fluid.

The silicon wafer cleaning fluid and method of the present inventionallow the silicon wafer surface to be etched simply with the etchingamount controlled to about several tens Å, and particularly about 20 to30 Å, and without damaging the smoothness of the surface. In addition,contamination with gold and other heavy metals can be reduced to notmore than 1/100. Since the level of contamination with heavy metals inthe controlled process of producing very large scale integrated circuitsis of the order of 10¹² atoms/cm², this cleaning can reduce the level to10¹⁰ atoms/cm² or less.

Further, the present silicon wafer cleaning fluid can adjust the etchingspeed and the etching amount by adjusting the composition of thecleaning fluid. For example, if the etching speed is adjusted to 20 to30 Å per min and the cleaning time is adjusted, one operation ofcleaning can bring the etching amount approximately to theabove-mentioned 10 to 20 Å and even when the cleaning is repeated twice,a smoothness substantially satisfactory to the production of very largescale integrated circuits can be obtained.

Further, the present silicon wafer cleaning fluid can be used even at atemperature of 10° C. or below by choosing a composition withoutlowering the above cleaning effect. Accordingly, even in the case wherethe problem of contamination of the clean room working atmosphere withacids is taken into consideration, by using the above cleaning fluid ata temperature of 10° C. or below, acids can be substantially preventedfrom evaporating from the cleaning fluid.

Since the present silicon wafer cleaning fluid is very high inwettability for silicon wafers, the surface can be kept wet until thewafer is rinsed with pure water, and fine particles in the cleaningfluid are allowed little to adhere to the silicon wafer surface, whichis advantageous in view of the measure against contamination with fineparticles.

Since the present silicon wafer cleaning fluid hardly allows the waferto be contaminated conversely with metal components in the cleaningfluid, the simple contact treatment of forming a thin layer of thecleaning fluid on the silicon wafer surface can achieve a satisfactorycleaning effect. As a result, said cleaning fluid can be used in anindividual-wafer cleaning apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship obtained in Example 3 betweenthe etching speed of the present cleaning fluid for P-type siliconwafers (crystal orientation: (100); and electric resistance: several Ωcm) and the concentration of a fluorine-containing surface-active agentcontained in said cleaning fluid.

FIG. 2 is a graph showing the relationship obtained in Example 9 betweenthe ¹⁹⁸ Au concentration of the cleaning fluid and the adsorption of ¹⁹⁸Au onto the silicon wafer onto the cleaning fluid.

DETAILED DESCRIPTION OF THE INVENTION

Now, the present invention will be described in detail.

Surface-Active Agents

As the surface-active agent, any of nonionic, amphoteric, and cationicsurface-active agents can be used so long as the surface-active agentdoes not contain any metal components, is excellent in permeability andwetting power for silicon wafers, in other words, good in wettability,and good in chemical stability in oxidizing acid fluids. Specificexamples include oxyethylene polyaddition products, such aspolyoxyethylene alkyl ethers whose alkyl group has generally 8 to 18carbon atoms and polyoxyethylene alkylphenol ethers whose alkyl grouphas generally 7 to 10 carbon atoms; imidazolinium betaine amphotericsurface-active agents; and fluorine-containing surface-active agents(e.g., perfluoroalkylamine oxides, which are nonionic,perfluoroalkylbetaines, which are amphoteric, and quaternary ammoniumsalts similar to perfluoroalkyltrimethylammoniums, which are cationic).More specifically, as an example of the oxyethylene polyadditionproduct, a polyoxyethylene nonylphenyl ether in which the molar numberof ethylene oxide is about 10, and specifically 9 to 13, is preferablyused. Among the above surface-active agents, fluorine-containingsurface-active agents having, in the molecule, a perfluoroalkyl groupwhose alkyl group has generally 4 to 10 carbon atoms are most preferablyused.

The concentration of the above surface-active agent in the cleaningfluid is 0.002 to 0.1% by weight, and preferably 0.002 to 0.05% byweight.

Acid Components

The concentration of HNO₃ is 35 to 65% by weight, and preferably 45 to55% by weight. On the one hand if the concentration of HNO₃ is less than35% by weight, a satisfactory removing effect for Au cannot be securedunless the concentration of HF or HCl is over than 0.5% by weight and onthe other hand if the concentration of HNO₃ is over 65% by weight, ahydrophilic abnormal film is liable to be formed on the wafer surfaceand the crystal completeness of the cleaned surface is difficult tocontrol.

The concentration of HF is 0.05 to 0.5% by weight, and preferably 0.1 to0.3% by weight. If the concentration of HF is less than 0.05% by weight,the removing effect for Au is lowered extremely while if theconcentration of HF is over 0.5% by weight, it becomes difficult tocontrol the etching amount, i.e., to secure the smoothness.

The concentration of HCl is 0.05 to 0.5% by weight, and preferably 0.1to 0.3% by weight. If the concentration of HCl is less than 0.05% byweight, the removing effect for Au is lowered extremely while if theconcentration of HCl is over 0.5% by weight, it becomes difficult tocontrol the etching amount, i.e., to secure the smoothness.

Other Additives

To the present silicon wafer cleaning fluid, may be added, for example,organic acids, such as acetic acid, in order to control the etching, andBr₂ or the like in order to strengthen the oxidizing power, which areadded in the range where they do not damage the cleaning effect.

Preparation of the Cleaning Fluid

In the present cleaning method, sometimes a certain way of preparing thecleaning fluid lowers the cleaning effect. The most preferred way ofpreparation is one wherein to an aqueous solution containing 55 to 70%by weight of nitric acid (concentrated nitric acid) are added an aqueoussolution containing a calculated amount of hydrofluoric acid, an aqueoussolution containing a calculated amount of hydrochloric acid, and anaqueous solution containing a calculated amount of a surface-activeagent (the water used in each aqueous solution being pure water) in thestated order. However the order of the hydrofluoric acid and thehydrochloric acid to be added can be reversed. If the surface-activeagent is added before the addition of the hydrofluoric acid or thehydrochloric acid, the cleaning effect is lowered. Even where thecleaning fluid is prepared in such a preferred way, after two hours, thecleaning effect of the cleaning fluid drops. Accordingly, it isdesirable that the cleaning fluid be prepared in a raw material liquidmixing tank connected directly to the cleaning apparatus immediatelybefore the cleaning is effected.

In the preparation, since satisfactory mixing is required in each stepof the addition, the mixing preparation apparatus including themechanism for weighing the raw material liquids becomes complicated andspecial care is needed for control. For this, a two-part-fluid mixingprocess is devised. According to this process, by only one mixingoperation, a good cleaning effect and an etching amount with goodreproducibility can be secured. The two-part-fluid mixing process is aprocess wherein the above four aqueous solutions (which may be inconcentrated form or diluted form) are suitably combined and mixed toform two solutions (i.e., two part-fluids) and these two solutions aremixed to prepare a cleaning fluid when or just before the cleaning iscarried out. Therefore, the present cleaning fluid may be provided inthe form of two fluids, for example, a combination of an aqueoussolution containing hydrochloric acid, hydrofluoric acid, and asurface-active agent with an aqueous solution of nitric acid (generallyin the form of concentrated nitric acid). Alternatively the presentcleaning fluid may be provided in the form of two fluids, one being anaqueous solution containing nitric acid (generally in the form ofconcentrated nitric acid) and hydrofluoric acid and one being an aqueoussolution containing hydrochloric acid and a surface-active agent. Thethus provided two fluids are mixed for use to prepared a cleaning fluid.Since these prepared fluids of the two-fluid type can be stored for along period of time, they can be prepared in advance.

By adjusting the concentrations of the above acid components, theetching power for the silicon wafer as well as the etching speed can beadjusted. In the case wherein the concentration of HNO₃ is high, bylowering the concentrations of HCl and HF, and in the case wherein theconcentration of HNO₃ is low, by increasing the concentrations of HCland HF, the desired etching power can be obtained. Although the cleaningeffect can be easily increased by increasing the concentration of HNO₃,environmental contamination by the HNO₃ atmosphere is liable to occurand therefore sufficient consideration is required for the exhaustmechanism. Therefore, in such a case wherein a large amount of the agentfluid is used for cleaning wafers by dipping, it is desirable to choosea composition in which the concentration of HNO₃ is low. Where thetemperature of the treatment fluid is lowered to 10° C. or below, sincethe evaporation of HNO₃ can be suppressed, the environmentalcontamination can be reduced considerably even if the cleaning effect isincreased by increasing the concentration of HNO₃.

Cleaning of Silicon Wafers

The cleaning of silicon wafers by the present cleaning fluid comprisesbringing the silicon wafer into contact with said cleaning fluid. Heavymetals on the silicon wafer are dissolved in the cleaning fluid to beremoved. Generally, desired cleaning is attained by bringing the siliconwafer in contact with the cleaning fluid for about 30 sec to 3 min.Specifically, by choosing a composition of the cleaning fluid, andbringing the silicon wafer in contact with the present cleaning fluid,for example, for about 1 min, the contamination of gold, copper, iron,and other heavy metals can be reduced to not more than 1/100, comparedwith that prior to cleaning, and if this cleaning is repeated twicesuccesively, the contamination can be reduced to not more than 1/1000.The contact (cleaning) temperature of the cleaning is about 0°-30° C.,and preferably about 5°-25° C. Even if the contact temperature is lowerthan 10° C., a satisfactory cleaning effect can be obtained as shown inExample 5 given below.

The heavy metals dissolved in the present cleaning fluid are lessadsorbed reversely onto the silicon wafer. For example, even if theconcentration of copper in the cleaning fluid reaches 100 ppb, theadsorption onto the silicon wafer is of the order of 10⁸ atoms/cm².Therefore, a satisfactory cleaning effect can be obtained by coveringthe silicon wafer only with a quite thin fluid layer of the cleaningfluid having, for example, a thickness of 0.1 to 1 mm.

Since the present cleaning fluid contains a surface-active agent, thecleaning fluid is quite high in wettability for silicon wafers, andtherefore the whole surface of the wafer can be covered with a thinfluid layer as mentioned above. Specifically, for example, similarly tothe spin coating method, a silicon wafer is rotatably held horizontally,and while the silicon wafer is spun with being kept horizontal, thepresent cleaning fluid is dropped little by little on the surface of thesilicon wafer, so that the whole surface of the wafer can be coveredwith the layer of the fluid (for instance, in the case of 6-inch siliconwafers, it is recommended that 10 to 20 cc of the cleaning fluid bedropped). It is possible that said silicon wafer surface is covereduniformly with said cleaning fluid for a prescribed period and thensuccessively is rinsed with pure water, and the rotational frequency isincreased to dry the silicon wafer surface by centrifuging so that aseries of cleaning treatments may be carried out (which is a so-calledpaddle treatment). The drying may be carried out by blowing a cleanatmosphere across the surface of the wafer. Alternatively, a series ofsteps for cleaning can be carried out such that the silicon wafer isheld vertically, the cleaning fluid is applied by pouring or sprayingfrom above to form a quite thin fluid layer of the cleaning fluid on thesilicon wafer surface, the silicon wafer is allowed to stand for aprescribed period of time and is then rinsed with pure water, and awarmed clean atmosphere is blown onto the silicon wafer surface. Also,the conventional dipping method can be applied, and in that case thesilicon wafer is dipped in the cleaning fluid and is then immediatelypulled out.

Generally, when a silicon wafer is cleaned with a cleaning fluid andbecomes dry once before rinsing with pure water, fine particles in thecleaning fluid adhere firmly to a dry region on the silicon wafer and itbecomes very difficult to remove those fine particles with pure rinsingwater. However, the present cleaning fluid is high in wettability forsilicon wafers and it is seldom that the silicon wafer becomes drybefore it is rinsed with pure water. Particularly, the cleaning fluidthat contains a fluorine-containing surface-active agent is high inwettability, and for example if only 0.01% of said surface-active agentis added, the silicon wafer can be brought to the pure water risingtreatment with almost all the surface of the silicon wafer being keptwet, and if 0.02% of said surface-active agent is added, positivewettability can be secured. Even in the case wherein a hydrocarbon typesurface-active agent is added, if the hydrocarbon type surface-activeagent is added in an amount twice the amount of a fluorine-containingsurface-active agent, satisfactory wettability of the cleaning fluid canbe obtained.

In the present cleaning fluid, the addition of a small amount of HF tothe main agent, HNO₃, allows the silicon wafer surface to be etchedwithout a spontaneous oxide film being formed, and although the amountof HCl to be added is very small, nitrosyl chloride enough to dissolveand remove metal contaminants on the wafer surface can be generated.

While nitrosyl chloride promotes etching, the added surface-active agentacts to suppress the etching and also the addition of a suitable smallamount of the surface-active agent controls the generation of nitrosylchloride to the necessary and sufficient amount. Thus, the speed ofetching the silicon wafer is controlled to about 20 to 30 Å/min, so thatthe smoothness of the cleaned wafer surface is not damaged.

EXAMPLES

Examples of the present invention will now be given below. The presentinvention is not limited by the following Examples at all.

In each of the Examples, % in the composition of the cleaning fluid(aqueous solution) is % by weight. Unless otherwise stated, thepreparation of the cleaning fluid according to the present invention wascarried out such that to nitric acid were added hydrofluoric acid,hydrochloric acid, and a surface-active agent in this order. Further,the cleaning was carried out such that a fluid layer of the cleaningfluid having a prescribed thickness was formed on the silicon wafer bythe casting method as mentioned above and retained for a prescribedperiod.

The cleaning effect of the present invention was confirmed by the tracermethod using a radioisotope element (RI). For example, to confirm thecleaning effect on contamination with gold, ¹⁹⁸ Au was used as a labelelement, the radiation counts of ¹⁹⁸ Au on the silicon wafer before andafter the cleaning were compared, the cleaning effect was studied byfinding the ratio of the radiation of gold adhered to the silicon waferafter cleaning to the radiation of gold adhered to the silicon waferbefore the cleaning (hereinafter referred to as the residual ratio (in%)). For contamination with copper and contamination with iron, ⁶⁴ Cuand ⁵⁹ Fe were used as label elements respectively, and similarly thecleaning effect was studied.

In the following, ¹⁹⁸ AU, ⁶⁴ Cu and ⁵⁹ Fe indicate gold, copper and ironlabeled with ¹⁹⁸ Au, ⁶⁴ Cu and ⁵⁹ Fe respectively. The term "cleaningtime" means the period wherein the fluid layer of the cleaning fluid wasretained.

Example 1

A P-type semiconductor silicon wafer (electric resistance: several Ω cm;and crystal orientation: (100)) contaminated with ¹⁹⁸ Au in an amount ofabout 10¹³ atoms/cm² was cleaned with a cleaning fluid (aqua regia orSC-2) that was conventionally considered to have a strong dissolvingaction on gold metal or a cleaning fluid of the present invention (thethickness of the fluid layer: about 1 mm), and the ¹⁹⁸ Au residualratios after the cleaning were compared. The results are shown in Table1.

                                      TABLE 1                                     __________________________________________________________________________                             Cleaning fluid                                                                        Each   Residual ratio                        Cleaning fluid                                                                        Composition      temperature                                                                           cleaning time                                                                        after cleaning                        __________________________________________________________________________    Aqua regia                                                                            37% hydrochloric acid                                                                     3 volumes                                                                          30° C.                                                                         10 min (A) 52%                                       70% nitric acid                                                                           1 volume            (B) 45%                               SC-2    37% hydrochloric acid                                                                     1 volume                                                                           70° C.                                                                         10 min (A) 72%                                       30% hydrogen peroxide                                                                     1 volume            (B) 65%                                       pure water  5 volumes                                                 Cleaning fluid                                                                        HNO.sub.3   55%  20° C.                                                                          1 min  (A) 0.6%                             of the present                                                                        HF          0.1%                  (B) 0.02%                           invention                                                                             HCl         0.1%                                                              perfluoroalkylamine                                                                       0.006%                                                            oxide                                                                         pure water  balance                                                   __________________________________________________________________________

In Table 1, (A) shows the ¹⁹⁸ Au residual ratio after the first cleaningand (B) shows the ¹⁹⁸ Au residual ratio after the second cleaning. Inthe case wherein aqua regia or SC-2 was used, immediately after thefirst cleaning, the silicon wafer was treated with dilute hydrofluoricacid (HF: H₂ O=1:10) and then the second cleaning was carried out (thethickness of the fluid layer: about 6 mm). On the other hand, in thecase wherein the cleaning fluid of the present invention was used, thesilicon wafer was not treated with diluted hydrofluoric acid, and afterthe rising with pure water, the second cleaning was carried out.

From the results of this Example, it is understood that, in the cleaningwith aqua regia or SC-2, if the silicon wafer surface was cleaned twicesuccessively, the contamination with gold was reduced at most only to aresidual ratio of 45%. In contrast, in the case wherein the cleaningfluid of the present invention was used, it was confirmed that evenwhere the contamination with gold on the silicon wafer was 10⁻⁻atoms/cm², the residual ratio after the first cleaning was 0.6%, i.e.,the ratio was of the order of 10¹⁰ atoms/cm² and by the secondsuccessive cleaning, the residual ratio could be brought to 0.02%, i.e.,the ratio could be brought to the order of 10⁹ atoms/cm². Further it wasconfirmed that the cleaning time in the case of the cleaning fluid ofthe present invention was as short as 1 min.

Example 2

The cleaning effects of a cleaning fluid not containing a surface-activeagent and a cleaning fluid containing a surface-active agent werecomparatively studied by cleaning P-type semiconductor silicon wafers(electric resistance: several Ω cm; and crystal orientation: <100>)contaminated with ¹⁹⁸ Au in an amount of about 10¹² atoms/cm². Thesilicon wafer was cleaned five times with the cleaning fluid having thecomposition shown in Table 2 (the thickness of the fluid layer: 0.5 to 1mm), thereby the stability of the cleaning effect was studied. Theconcentrations of the acids in each of the cleaning fluids in Table 2were adjusted concentrations where the etching amount for the siliconwafer became 15 Å on average when the cleaning was effected at roomtemperature (20° C.) for 1 min. The results are shown in Table 2.

From Table 2, it could be confirmed that the cleaning effect of thecleaning fluid not containing a surface-active agent is unstable anddifficult to control while the cleaning effect of any of the cleaningfluids of the present invention is stable where certain surface-activeagents are used although there is a little difference in cleaning effectdepending on the type of the surface-active agent used. It was alsoconfirmed that the cleaning fluid containing a surface-agent having aperfluoroalkyl group whose alkyl group having 8 carbon atoms in themolecule is particularly high in cleaning effect and stable in cleaningeffect. These results show that the results of cleaning obtained by thepresent invention are not scattered and are high in reliability.

                  TABLE 2                                                         ______________________________________                                                              Type and                                                       Composition of acids                                                                         concentration of                                                                          Residual ratio                              Cleaning                                                                             of cleaning fluid (%)                                                                        surface active                                                                            after cleaning                              fluid No.                                                                            HNO.sub.3                                                                             HCl    HF    agent     (%)                                     ______________________________________                                        201    55      0.1    0.03  --        first: 0.5                                                                    second: 3.5                                                                   third: 1.8                                                                    fourth: 4.0                                                                   fifth: 1.2                              202    55      0.1    0.1   polyoxyethylene                                                                         first: 1.0                                                          nonylphenyl                                                                             second: 1.1                                                         ether, 0.01%                                                                            third: 1.0                                                                    fourth: 0.8                                                                   fifth: 0.9                              203    55      0.1    0.1   imidazolinium                                                                           first: 1.9                                                          betaine, 0.01%                                                                          second: 1.7                                                                   third: 1.6                                                                    fourth: 1.5                                                                   fifth: 1.9                              204    55      0.1    0.1   perfluoroalkyl                                                                          first: 0.5                                                          quaternary                                                                              second: 0.4                                                         ammonium salt,                                                                          third: 0.6                                                          0.01%     fourth: 0.5                                                                   fifth: 0.7                              ______________________________________                                    

Example 3

In a cleaning fluid of the present invention where the composition ofthe acid components were made up of 55% of HNO₃, 0.1% of HF, and 0.1% ofHCl, the concentration of a surface-active agent, a perfluoroalkylquaternary ammonium salt whose alkyl group has 6 carbon atoms, wasvaried to prepare several cleaning fluids, and with respect to each ofthe thus prepared cleaning fluids, the etching speed at 25° C. wasdetermined. The silicon wafers that were cleaned were P-typesemiconductor silicon wafers (electric resistance: several Ω cm; andcrystal orientation: (100)). The cleaning was carried out at 25° C. for1 min. The thickness of the fluid layer was about 1 mm. The results areshown in FIG. 1.

FIG. 1 shows that in order to prepare a cleaning fluid having an etchingspeed of about 20 to 30 Å/min suitable for cleaning silicon wafers, itis desirable to bring the concentration of the surface-active agent inthe cleaning fluid to fall in the range of 0.002 to 0.05%.

Example 4

In a cleaning fluid of the present invention that contained 0.02% of aperfluoroalkylbetaine whose alkyl group had 7 carbon atoms, theconcentrations of the acid components were varied to study the cleaningeffect for ¹⁹⁸ Au contamination (the thickness of the fluid layer: about1 mm). The results are shown in Table 3. The ¹⁹⁸ Au residual ratio inTable 3 was obtained by cleaning ¹⁹⁸ Au contamination (10¹² atoms/cm²)on P-type semiconductor silicon wafers (electric resistance: several Ωcm; and crystal orientation: (100)) with each of the cleaning fluidsmade at 25° C. for 40 sec.

                  TABLE 3                                                         ______________________________________                                        Composition of                                                                cleaning fluid (%)                                                                                          Surface                                                                              Residual ratio                           Cleaning                      active of .sup.198 Au after                     fluid No.                                                                            HNO.sub.3                                                                              HF      HCl   agent  cleaning (%)                             ______________________________________                                        401    65       0.4     0.1   0.02   0.2                                      402    60       0.4     0.1   0.02   0.6                                      403    55       0.4     0.1   0.02   0.8                                      404    50       0.4     0.1   0.02   3.1                                      405    45       0.4     0.1   0.02   5.2                                      406    40       0.4     0.1   0.02   17                                       407    35       0.4     0.1   0.02   46                                       408    45       0.4     0.4   0.02   3.8                                      409    35       0.5     0.5   0.02   4.2                                      410    65       0.05    0.5   0.02   0.6                                      411    65       0.4     0.05  0.02   1.3                                      ______________________________________                                    

From Table 3, it can be understood that where the concentration of HNO₃is increased, the cleaning effect on ¹⁹⁸ Au is also increased, and ifthe concentration of HNO₃ is low, by increasing the concentration of HFor HCl a satisfactory cleaning effect can be obtained.

Example 5

In the cases wherein the temperature of the cleaning fluid of thepresent invention was 10° C. or below (the thickness of the fluid layer:about 1 mm), the cleaning effect on ¹⁹⁸ Au contamination in an amount of10¹² atoms/cm² on N-type semiconductor silicon wafers (electricresistance: several Ω cm; and crystal orientation: (100)) was studied.The results are shown in Table 4. The surface-active agent used was aperfluoroalkyl quaternary ammonium salt whose alkyl group had 8 carbonatoms. The cleaning was carried out for 40 sec.

                  TABLE 4                                                         ______________________________________                                        Composition of                                                                cleaning fluid (%)       Residual ratio                                                                     Surface                                                                              of .sup.198 Au After                     Cleaning                      active cleaning (%)                             fluid No.                                                                            HNO.sub.3                                                                              HF      HCl   agent  5° C.                                                                        10° C.                      ______________________________________                                        501    55       0.1     0.1   0.02   7.5   4.8                                502    55       0.2     0.1   0.02   4.1   2.1                                503    55       0.2     0.2   0.02   2.2   1.0                                504    55       0.3     0.2   0.02   1.0   0.6                                ______________________________________                                    

From Table 4, it can be understood that if the temperature of thecleaning is lowered to below 10° C., only by increasing theconcentrations of HF and HCl a little, the cleaning fluid can provide asatisfactory cleaning power. In addition, the concentration of HNO₃ inthe clean booth where the cleaning was carried out at 5° C. was thealmost same as that in the clean room adjacent to the clean room, thatis, the concentration of HNO₃ in the clean booth was 1 ppb or below. Incontrast, in the case where the same cleaning was carried out at 25° C.,the concentration of HNO₃ in the clean booth reached 18 ppb andtherefore sufficient exhaustion was required.

Example 6

In the preparation of the cleaning fluid of the present invention, theorder of the addition of raw material liquids to nitric acid was variedand the influence of the cleaning effect of that cleaning fluid on ¹⁹⁸Au contamination (10¹² atoms/cm²) on N-type semiconductor silicon wafers(electric resistance: several Ω cm; and crystal orientation: (100)) wasexamined. The cleaning fluid was composed of 55% of HNO₃, 0.1% of HF,0.1% of HCl, 0.02% of a surface-active agent. The surface-active agentwas a perfluoroalkyl quaternary ammonium salt whose alkyl group had 8carbon atoms. The cleaning (the thickness of the cleaning fluid: about 1mm) was carried out at 20° C. for 1 min. The results are shown in Table5. The cleaning fluid Nos. 601 and 602 marked with * were preferablemodes of the present invention.

                  TABLE 5                                                         ______________________________________                                                                     Residual ratio                                   Cleaning                     of .sup.198 Au after                             fluid No.                                                                            Order of addition to nitric acid                                                                    cleaning (%)                                     ______________________________________                                         601*  hydrofluoric acid, hydrochloric acid,                                                               0.4                                                     and surface active agent                                                602*  hydrochloric acid, hydrofluoric acid,                                                               0.5                                                     and surface active agent                                               603    surface active agent, hydrofluoric acid,                                                            4.4                                                     and hydrochloric acid                                                  604    surface active agent, hydrochloric acid,                                                            3.3                                                     and hydrofluoric acid                                                  605    hydrofluoric acid, surface active agent,                                                            2.5                                                     and hydrochloric acid                                                  606    hydrochloric acid, surface active agent,                                                            1.4                                                     and hydrofluoric acid                                                  ______________________________________                                    

The addition of these components was carried out such that after thefirst addition, stirring was well performed, then the second additionwas carried out, stirring was well performed, and the third addition wasperformed, followed by stirring well. It can be understood that wherethe addition of a surface active agent precedes the addition ofhydrofluoric acid or hydrochloric acid, the cleaning effect is lowered alittle.

Example 7

Fluid A and Fluid B shown in Table 6 were prepared separately, then theywere mixed for use to prepare the cleaning fluids of the presentinvention, and the cleaning effect of the cleaning fluids was studied.The cleaning effect of the cleaning fluids on ¹⁹⁸ Au contamination (10¹²atoms/cm²) on an N-type semiconductor silicon wafer (electricresistance: several Ω cm; and crystal orientation: (100)) and theetching amount by the cleaning fluids were studied. The cleaning (thethickness of the fluid layer: about 1 mm) was carried out at 20° C. for1 min. The surface-active agent used was a perfluoroalkyl quaternaryammonium salt whose alkyl group had 9 carbon atoms.

                                      TABLE                                       __________________________________________________________________________    Composition before mixing (%)                                                                             Residual ratio                                    Fluid A: 78 Fluid B: 22                                                                           Composition after                                                                     of .sup.198 Au after                                                                 Etching                                    volumes     volumes mixing (%)                                                                            cleaning (%)                                                                         amount (Å)                             __________________________________________________________________________    Preparation                                                                         HNO.sub.3 (70)*                                                                     HF (0.45)                                                                             HNO.sub.3 (55)                                                                        0.5    18                                         Example 1   HCl (0.45)                                                                    surface-active                                                                        HF (0.1)                                                              agent (0.09)                                                      Preparation                                                                         HNO.sub.3 (70)*                                                                     HCl (0.45)*                                                                           HCl (0.1)                                                                             0.4    17                                         Example 2                                                                           HF (0.13)                                                                           surface-active                                                                        surface-active                                                        agent (0.09)                                                                          agent (0.02)                                              __________________________________________________________________________     (Note) *The balance was water.                                           

The compositions of the cleaning fluids and the cleaning conditions ofthis Example are the same as those of Example 6 and it can be understoodthat the cleaning effect by this two-fluid mixing method is notdifferent from the best cleaning effect of Example 6 where thecomponents were added successively and that the etching amount iscontrolled well.

Example 8

After such a treatment as reactive ion etching and ion implantation,since crystal defects are formed on the silicon wafer surface, sometimesit is necessary to etch the surface to several tens Å or more to removesaid defects. The lowering of the smoothness of the silicon wafersurface formed by such etching are caused in many cases due to thedifference in etching speed by the cleaning fluid depending on thecrystal orientation. Therefore, two types of silicon wafers different incrystal orientation, i.e., an N-type semiconductor silicon wafer(electric resistance: several Ω cm; and crystal orientation: (100))(hereinafter referred to as (100)) and an N-type semiconductor siliconwafer (electric resistance: several Ω cm; and crystal orientation:(111)) (hereinafter referred to as (111)) were cleaned with the presentcleaning fluid (composed of 55% of HNO₃, 0.1% of HF, 0.1 of HCl, and0.02% of a perfluoroalkyldimethylhydroxyethyl quaternary ammonium saltwhose alkyl group had 8 carbon atoms) and the etched amounts of thesilicon wafer surfaces after the cleaning were compared. The cleaning(the thickness of the fluid layer: about 1 mm) was carried out at 25° C.for 10 min.

The etched amounts of the (100) and the (111) were scattered between 120Å and 180 Å and there was no significant difference between them. Fromthis Example, it can be confirmed that the etching action of the presentcleaning fluid is not affected by the crystal orientation of the siliconwafer.

Example 9

The adsorption of ¹⁹⁸ Au onto the silicon wafer surface from thecleaning fluid of the present invention was determined with theconcentration of ¹⁹⁸ Au varied in the cleaning fluid. The thickness ofthe fluid layer was about 1 mm. The results are shown in FIG. 2.

From FIG. 2, it can be understood that if there is gold at a higherconcentration in the present cleaning fluid, the amount of gold in thecleaning fluid which will be adsorbed onto the silicon wafer is quitesmall.

Example 10

In a laboratory device having a structure similar to the spin coater forphotoresist, a 6-inch silicon wafer only one surface of which wascontaminated with ¹⁹⁸ Au in an amount of 10¹² to 10¹³ atoms/cm² was setso that the contaminated surface was upward while the silicon wafer wasrotated, about 5 cc of a cleaning fluid containing 55% of HNO₃, 0.2% ofHF, 0.2% of HCl, and 0.02% of a polyoxyethylene alkylnonyl ether whosealkyl group had 9 carbon atoms was dropped to be spread throughout thesilicon wafer, then the rotation was stopped and the silicon wafer wasallowed to stand for 1 min. The thickness of the fluid layer at thattime was about 0.5 mm. Thereafter the silicon wafer was rotated againwhile pure water was poured to rinse the silicon wafer for 30 sec, andthen after the silicon wafer was rotated at a higher speed to be dried,the ¹⁹⁸ Au residual ratio on the silicon wafer surface was determined.With respect to a 6-inch silicon wafer contaminated with ⁶⁴ Cu or ⁵⁹ Fe,similarly the cleaning was carried out, and the ⁶⁴ Cu or ⁵⁹ Fe residualratio was found. The results showed that the ¹⁹⁸ Au residual ratio was0.6%, the ⁶⁴ Cu residual ratio was 0.2%, and the ⁵⁹ Fe residual ratiowas 0.1% or less.

Additionally, in the case where the above drying was carried out byblowing clean nitrogen, good results, similar to the above results, ofthe removal of ¹⁹⁸ Au, ⁶⁴ Cu, and ⁵⁹ Fe contamination were obtained.

Example 11

A silicon wafer contaminated similarly to Example 9 with ¹⁹⁸ Au, ⁶⁴ Cu,or ⁵⁹ Fe was held approximately vertically and the same cleaning fluidas that of Example 9 was sprayed from above onto the silicon wafersurface. After confirming that the whole surface of the silicon waferwas wetted with the cleaning fluid, the silicon wafer was allowed tostand for 1 min (at that time the thickness of the fluid layer was about0.5 mm), the silicon wafer rinsed with pure water, and high-purity airthat had been subjected to microfiltration was blown to dry the siliconwafer. The heavy metal residual ratios of the silicon wafer surfaceswere determined and the ¹⁹⁸ Au residual ratio was 0.4%, the ⁶⁴ Curesidual ratio was 0.2%, and the ⁵⁹ Fe residual ratio was 0.1% or less.

What is claimed is:
 1. A silicon wafer cleaning method, comprisingbringing a fluid for cleaning silicon wafers consisting essentially of35 to 65% by weight of HNO₃, 0.05 to 0.5% by weight of HF, 0.05 to 0.5%by weight of HCl, 0.002 to 0.1% by weight of a surface-active agent, andwater in contact with the silicon wafer surface, said fluid having beenprepared by mixing the components in a manner such that said surfaceactive agent is not added to the HNO₃ and water before the addition ofaqueous HF or aqueous HCl to the HNO₃ and water, said contact occurringwithin 2 hours from the preparation of said fluid.
 2. The silicon wafercleaning method of claim 1, wherein said surface-active agent comprisesa compound having a perfluoroalkyl group in the molecule.
 3. The siliconwafer cleaning method of claim 1, wherein the contact temperature is 0°to 30° C.
 4. The silicon wafer cleaning method of claim 3, wherein thecontact temperature is 10° C. or below.
 5. The silicon wafer cleaningmethod of claim 1, wherein said contact is carried out by covering thewhole surface of the silicon wafer with a fluid layer of said cleaningsilicon wafer cleaning fluid.
 6. The silicon wafer cleaning method ofclaim 5, wherein the thickness of the fluid layer is 0.1 to 1 mm.
 7. Thesilicon wafer cleaning method of claim 5, wherein the fluid layer isformed by dropping said fluid for cleaning silicon wafers on the surfaceof a silicon wafer that is spun in a horizontal plane.
 8. The siliconwafer cleaning method of claim 5, wherein the fluid layer is formed bypouring and casting said silicon wafer cleaning fluid on the surface ofthe silicon wafer that is held vertically.
 9. The silicon wafer cleaningmethod of claim 5, wherein the fluid layer is formed by spraying saidsilicon wafer cleaning fluid onto the surface of the silicon wafer thatis held vertically.
 10. The silicon wafer cleaning method of claim 1,further comprising rinsing with pure water.
 11. The silicone wafercleaning method of claim 1, where said surface-active agent is afluorine-containing surface active agent.
 12. A silicon wafer cleaned bya cleaning method which comprises bringing a fluid consistingessentially of 35 to 65% by weight of HNO₃, 0.05 to 0.5% by weight ofHF, 0.05 to 0.5% by weight of HCl, 0.002 to 0.1% by weight of asurface-active agent, and water in contact with the silicon wafersurface, said fluid having been prepared by mixing the components in amanner such that said surface active agent is not added to the HNO₃ andwater before the addition of aqueous HF or aqueous HCl to the HNO₃ andwater, said contact occurring within 2 hours from the preparation ofsaid fluid.